Good genes aren't the only way to gain an evolutionary advantage. You can also enlist other organisms to fight off threats - and then pass those helpful symbiotic partners on to your offspring. It's an all new kind of evolution.

In all previous well-documented examples of natural selection, favorable genes provided organisms with traits necessary to maximize their chances of surviving to reproductive age and passing on their advantages to their offspring. As such, it was taken as read that genes were the only engines of natural selection, but a new study at the University of Rochester in New York shows that's not necessarily the case.

Biologist John Jaenike and his colleagues discovered a case where bacteria latched onto an organism, helped its host survive to reproductive age, and then got passed onto its children. That's pretty much exactly the same way genes operate in an evolutionary context. It's the first time this has been documented in the wild, but Jaenike thinks it's probably a frequent phenomenon that we simply haven't noticed before.

The biologists focused on a fly that can be rendered sterile by a destructive parasitic worm known as a nematode. The nematodes burrow inside the female flies' skin while they are young and make it impossible for the flies to produce eggs later. However, if the fly is also infected with the bacteria Spiroplasma, the nematode finds the fly an inhospitable host and is unable to sterilize it. This bacterial infection is then passed onto the fly's children, providing ready-made protection from the nematode and conferring a massive reproductive advantage.

Testing preserved flies from the early 1980s, the biologists found that only 10% possessed the helpful Spiroplasma. By 2008, that number had jumped to a staggering 80%. As Jaenike explains, the impact of this on the flies' wellbeing has been enormous:

"These flies were really getting clobbered by nematodes in the 1980s, and it's just remarkable to see how much better they are doing today. The spread of Spiroplasma makes me wonder how much rapid evolutionary action is going on beneath the surface of everything we see out there."

Jaenike suspects the flies evolved in this particular way because nematodes only recently colonized North America, posing an entirely new and potentially cataclysmic threat to the species. Without any natural defenses of their own, the flies stumbled upon Spiroplasma as an effective defense against nematode sterilization, and so they co-opted the genetic advantage of the bacteria species and passed it on to their own offspring. Without any change at all to their own genes, the flies managed to adapt and survive.

Beyond shedding light on a new evolutionary mechanism, this study could also be helpful in controlling human disease. Nematodes are carriers of some nasty human diseases, like river blindness and elephantiasis, and it's possible we can learn a trick or two in preventative medicine from these flies and Spiroplasma.